Library / Mechanical / MEC-21
Mechanical
Friction limit
Maximum static friction force, or the kinetic friction force when sliding.
- Ref
- MEC-21
- Rev
- v1
- Field
- Mechanical
- Result
- Fmax [N]
Static friction adjusts up to the limiting value μsN; sliding contact has kinetic friction μkN opposing relative slip. N is the compressive normal reaction. The calculator gives the limiting value for the coefficient entered.
Friction can resist sliding up to a limit that depends on how hard the surfaces are pressed together.
Calculator
- Angle of friction arctan μ21.8 °
- Friction as a fraction of N0.4
What drives the result
Each variable is moved across its typical range while the others stay at the values above. The longest bar is the key driver.
- Kinetic friction is normally lower than the static limit, which is why a skid is harder to recover from than a grip.
- A block starts to slide on a slope when the slope exceeds the angle of friction.
Chart data as tables
Ranked variables
| Variable | Typical range | Lowest result | Highest result | Swing (N) | Local step | Change in result |
|---|---|---|---|---|---|---|
| N Normal reaction | 10 to 20,000 N | 4 | 8,000 | 7,996 | +10% | +10 % |
| μ Friction coefficient | 0.05 to 1.5 | 40 | 1,200 | 1,160 | +undefined% | — |
Sweep of Normal reaction
| Normal reaction (N) | Friction force limit (N) |
|---|---|
| 10 | 4 |
| 25.86 | 10.34 |
| 66.87 | 26.75 |
| 172.9 | 69.17 |
| 447.2 | 178.9 |
| 1,156 | 462.6 |
| 2,991 | 1,196 |
| 7,734 | 3,094 |
| 20,000 | 8,000 |
Sweep one variable
Variables and how each one contributes
Friction force limit
What the equation gives you. Everything else feeds it.
Friction coefficient
Static (limit) or kinetic (sliding) coefficient. Directly proportional.
Normal reaction
Compressive contact force. Directly proportional.
Worked example with the values above
- μ = 0.4; N = 800 N.
- Multiply: 0.4 × 800.
- Fmax = 320 N.
Assumptions and limits
- Dry contact.
- Friction coefficient independent of contact area.
Common mistakes
- Using the weight in place of the normal reaction on a slope or when other loads act.
- Applying the limit when the contact is not about to slip.
Statistics and uncertainty
Friction coefficients vary by 20% or more with surface state, so use ranges not single values.
Where it is used
- Brakes, clutches and tyre grip.
- Equipment holding and sliding checks.
Maths you need
- Normal reaction: force perpendicular to the contact
- Equilibrium: ΣF = 0